1. Field of The Invention
The present invention relates to an improvement of a control apparatus for a drive system of a hybrid vehicle.
2. Description of Related Art
There is known a hybrid vehicle drive system including: a differential device which comprises a first differential mechanism and a second differential mechanism and which comprises four rotary components; an engine, a first electric motor, a second electric motor and an output rotary member which are respectively connected to said four rotary components; and a plurality of coupling elements. JP-2011-98712 A1 discloses an example of a hybrid vehicle transmission system configured to switch the hybrid vehicle drive system to a selected one of a plurality of vehicle drive modes, according to a selected one of different combinations of operating states of the coupling elements.
In the prior art hybrid vehicle drive system constructed as described above, the output rotary member is rotated in a reverse direction to drive a hybrid vehicle in a reverse direction, with an operation of the first or second electric motor in a negative direction. When the engine is started as a result of an increase of a required vehicle drive force while the hybrid vehicle is driven in the reverse direction, there is a risk of reversal of the rotating direction of the output rotary member from the reverse direction to a positive or forward direction and a rise of its rotating speed with a rise of the operating speed of the engine, depending upon a specific combination of the operating states of the coupling elements, so that the hybrid vehicle may not be adequately driven in the reverse direction. This problem was first discovered by the present inventor in the process of an intensive study in an effort to improve the performance of the hybrid vehicle.